Scientific Method —

Measuring the strength of individual graphene sheets

Researchers use an atomic force microscope to measure the strength of …

Research into graphene—a sheet of carbon a single atom thick—has yielded measurements of the intrinsic strength of the material, showing that graphene is the strongest material ever encountered. Graphene is elusive in that it is most frequently encountered as a component of bulk, three-dimensional graphite, or the hot-topic of materials science: carbon nanotubes (simply a rolled up sheet of graphene). Deriving the intrinsic mechanical properties of graphene from either of these two more common derivatives is problematic due to the complicated geometry considerations of carbon nanotubes and the defects in bulk graphite.

Defects act as stress concentration sites in a material and, as a consequence, have a large impact on its strength, especially in brittle materials—a principle known as the Griffith Theory. Reducing the size of a material reduces the defect density; the basic principle used for high-strength, fiber-reinforced composites. The defect-free nature of a sheet of pure graphene should allow it to approach the theoretical strength of the material, as defined by the bonding forces between atoms.

To measure the elastic modulus (stiffness) and the strength of graphene, researchers fabricated a series of "wells" one micron deep in a silicon substrate, and deposited graphene so that it was positioned over the wells. The graphene was anchored around the rim of the well, and an atomic force microscope was used to press the graphene into the well and measure the strength of the material.

Atomic force microscopy relies on piezoelectric materials and very precise voltage control and measurement to manipulate an arm-and-tip assembly to measure forces from atomic interactions. Think of it like a record player where, instead of moving around on grooves to produce music, the atomic force microscope moves around on individual atoms to produce an image. The same arm-and-tip apparatus can also be used to measure forces and manipulate individual atoms and molecules via electrostatic forces.

The graphene registered an elastic modulus of 1.0 terapascals and a strength of 130 gigapascals, putting it at the top of the heap as far as raw strength in a material is concerned. Non-linear elastic behavior was also reported, a complicated phenomenon not usually considered when dealing in bulk materials.

The researchers went so far as to suggest the use of graphene in the mythical space elevator application that could enable cheap space travel for the masses. One must put this in perspective though. Carbon nanotubes—the previous record holder for measured strength in a material—have been studied for close to two decades without clear benefits to macroscopic structural materials. The devil is always in the details.